Thermosetting, or curable, coating compositions are widely used in coatings operations. In automotive coatings, in particular, thermoset coatings provide durable finishes. Automotive coatings include primers and topcoats, which may be single layer enamels or two layer basecoat/clearcoat systems. The primer may be applied either as a first coating layer or over another layer, for example over an electrocoat primer layer. The topcoat is then usually applied directly over the primer layer.
While thermosetting compositions provide desirable coating properties, there are a number of concerns that arise with thermosetting coating compositions. One consideration is the curing conditions needed to achieve sufficient crosslinking of the film. In general, higher temperatures and longer times at the curing temperature increase the manufacturing costs of the coated article. Another concern in some cases is that undesirable by-products of the curing reaction are generated. For example, blocked curing agents release the blocking agents as volatile organic compounds that constitute emissions regulated by various government rules. It is also important that the crosslinks that are formed are suitable for providing long life to the coating under the particular conditions to which the coated article will be exposed.
Different crosslinking mechanisms may be employed in thermosetting coatings. One curing mechanism utilizes a melamine formaldehyde resin curing agent in the coating composition to react with hydroxyl groups on the resin. This curing method provides good cure at relatively low temperatures (e.g., 250.degree. F. with a blocked acid catalyst, or even lower with an unblocked acid catalyst), but the crosslink bonds contain undesirable ether linkages and the resulting coatings may provide poor overall durability under certain service conditions. In an alternative curing method, polyisocyanate curing agents may be reacted with amine or hydroxyl groups on the resin. This curing method provides desirable urea or urethane crosslink bonds, but it also has several disadvantages. In order to prevent premature gelation of the coating composition, the polyisocyanate must either be kept separate from the resin in what is known in the art as a two-package or two-pack coating system or else the highly reactive isocyanate groups on the curing agent must be blocked (e.g., with an oxime or alcohol). Blocked polyisocyanates, however, require high temperatures (e.g., 150.degree. C. or more) to unblock and begin the curing reaction. The volatile blocking agents released during cure can adversely affect coating properties, as well as increasing the volatile organic content for the composition.
There is thus a need in the art for coating compositions that could provide desirable urethane crosslink linkages, but avoid the problems that accompany the use of polyisocyanate curing agents.
Coating compositions comprising carbonate curing agents and primary amine-functional crosslinkable resins have been proposed for electrocoat primers in December et al., U.S. Pat. No. 5,431,791. In the electrocoat bath, the primary amine groups are salted and rendered unreactive with the carbonate groups of the carbonate compound. When the coating is deposited onto the conductive substrate, the primary amine groups are regenerated from the salt and are once more reactive toward the carbonate curing agent. This method of achieving package stability, however, is unsuitable for solventborne and non-cathodic waterborne compositions.